Mirrors have been known for a considerable period of time. The reflective properties of a mirror have been used in a vast variety of situations. For instance a headlight on an automobile has a mirror on the back portion of the light. This mirrored surface reflects light from the headlight in a defined direction, i.e. toward the road. Mirrors have been used to direct light in a great variety of well known situations.
Mirrors also come in two general categories, i.e. back surfaced and front surfaced. A back surfaced mirror is perhaps the most common form of mirror. It typically includes a piece of glass with a substance such a paint on the back side. Reflected light therefore needs to pass through the glass before it is reflected away from the mirror. Once reflected, the light passes back out through the glass. Passing through the glass (twice) creates ambiguity in the direction of the reflection. The glass bends the light to a degree, creating an imperfect reflection.
In some applications reflective accuracy is imperative. A front surfaced mirror improves reflective accuracy. On such a mirror, the front surface is as smooth as is humanly possible to increase reflective accuracy. The reflective surface is the first surface light contacts. The front surface immediately reflects the light before it passes through a piece of glass, removing ambiguities inherent when the light passes through glass. Aluminum, Al, is commonly used as the reflective surface on front surface mirrors.
A unique situation for use of mirrors arises when the mirror reflects light rays in and near the infrared and ultraviolet bands. Such a situation may arise for instance when laser beams are reflected. The problem is that aluminum is a reactive metal when in the presence of such light. A chemical reaction with aluminum and surrounding material creates ambiguities on the surface of the mirror, injuring or destroying the accuracy of the reflection. Aluminum is also easily scratched further preventing it from being an ideal material for a front surface mirror. The high reflectivity of aluminum, however, continues to make aluminum a substance of choice for front surface mirrors.
Various methods have unsuccessfully attempted to solve the problems encountered with reflecting light in the infrared and ultraviolet bands. Rear surface mirrors minimize reactivity, but are often not suitable due to the inaccuracies from the glass. Front surface mirrors, are somewhat useful, but are quickly destroyed with reactivity, abrasion and other degradation of the aluminum. Sometimes a thin layer of compound such as silicon oxide or silicon dioxide is used to lower the rate of degradation of the reflective surface on a front surface mirror. Such a layer, however, often absorbs light in the infrared or ultraviolet bands, making the mirror useless when the light to be reflected is in such ranges. Moreover, the coating creates ambiguities much like glass as the film thickness approaches the wavelength of light interference from back to front makes the reflection wavelength selective.
Various mirror applications need a mirror with excellent reflectivity properties in and near the infrared and ultraviolet bands similar to that of aluminum. The reflective surface, however, needs to be abrasion resistant, corrosion resistant and resist other chemical reactivity. Preferably these properties would more closely resemble the degradation properties of stainless steel. Further, the mirror needs to have the reflective accuracy of a front surface mirror.